The present exemplary embodiment relates to a braking system for a vehicle. It finds particular application in conjunction with a foot valve for a braking system, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
A typical vehicle braking system for a straight truck, bus, tractor, or trailer, includes a source of pressurized air along with valves for selectively directing the air to brake chambers at the wheels of the vehicle. Vehicle air brake systems typically include a primary circuit, which is often used for driven wheels, and a secondary circuit, which is often used for non-driven wheels. The vehicle has a dual brake valve, or foot brake valve (FBV), which is a valve in both the primary circuit and the secondary circuit that is controlled by the foot pedal (brake pedal) of the vehicle in response to driver demand for braking. The dual brake valve is supplied with high pressure air from one or more reservoirs. When the valve is actuated by driver applied force on the brake pedal, this high pressure air is directed into the primary and secondary braking circuits of the vehicle. Many such vehicle braking systems provide an antilock braking system (ABS) function, by which an electronic control unit (ECU) selectively releases and applies braking at individual wheels to prevent wheel lockup.
Some vehicle braking systems also provide an automatic traction control (ATC) function. In one aspect of ATC, an ECU selectively applies braking at individual wheels to match wheel speeds side to side to help control wheel spin that occurs in response to driver demand via the accelerator pedal. This control is typically effected by controlling a wheel end modulator associated with the wheel. The modulator provides an air flow path to the wheel that can be rapidly opened or closed by a solenoid under the control of the ECU.
In order to provide the ATC function, high pressure air typically is made available at the wheel end modulators in the absence of driver demand. This is commonly done by having a constant supply of high pressure air from a reservoir to an ATC solenoid that is associated with the modulators on the driven axle. In an ATC event, the ATC solenoid is energized under the control of the ECU to direct the high pressure air from the reservoir to the modulators. The modulators are then controlled by the ECU selectively to apply and release braking force to the wheels, to control any wheel spin.
As one example, FIG. 1 shows schematically some portions of a prior art vehicle braking system that is operative to provide ATC on a straight truck, or bus. The system is also ABS effective. The system includes a foot brake valve (FBV) that normally provides driver demand proportioned air pressure as a control air flow to a relay valve (designated ATC) associated with the driven wheels of the vehicle. The relay valve includes an ATC solenoid that is controlled by an ECU. The ATC solenoid is normally de-energized, in which case driver demand proportioned air is used as the control pressure on the relay valve, to control the flow of air from the reservoir to the rear axle modulators. When the ECU determines that the traction control function is desired, the ECU energizes the ATC solenoid on the relay valve, which opens the relay valve immediately, in spite of the lack of driver demand controlled pressure. Reservoir air pressure is provided to rear modulators M. The rear modulators M, under the control of the ECU, use that air to modulate brake pressure in the brake actuators, thus controlling the spinning of the rear wheels.
Some vehicles with dual brake valves also have a brake valve actuator (BVA), which is an actuatable device interposed between the valve and the brake pedal that applies enough force to the brake valve so that the brake valve's output (delivery pressure to the primary and secondary circuits). The BVA is operated, for example, (piloted by, or receives a pneumatic control signal from) by (a) a pneumatic on/off control valve actuated by the driver's hand like a switch, or (b) an on/off solenoid valve controlled by an on/off electrical switch operated by the driver's hand. The BVA is used to conduct a pre-trip inspection, in which the brakes are set and held in an applied condition so that the driver can check the braking system of the vehicle without having to be in the cab pressing on the brake pedal. U.S. Pat. No. 6,659,244 shows the use of a brake valve actuator in a vehicle air braking system.
It is also known to control a brake valve actuator (BVA) of a vehicle with an ECU to provide ATC, yaw stability (YS) and ESP functions for the vehicle. These functions require supply (reservoir) air to be available to the vehicle's wheel end modulators in the absence of driver demand (brake pedal movement). This is effected by having the ECU control the BVA to actuate the dual brake valve without driver intervention.
An example of such a system is disclosed in U.S. Pat. No. 7,520,572, wherein the ATC function is provided by making braking pressure available at the driven wheels only of the vehicle. This may be done by blocking the modulators for the non-driven wheels so that no braking effect is provided at the non-driven wheels. At the same time, the modulators for the driven wheels are controlled to enable selective braking, under control of the ECU, of the driven wheels. The ATC function in this manner can be provided for a straight truck or bus, or for a tractor and trailer, without the need for an ATC solenoid. For a tractor and trailer, there is a need to be able to stop pressure delivery to the trailer, which is typically met by a standard modulator controlling the trailer delivery directly from the brake ECU.
That is, since the modulators for the non-driven wheels can hold, and not allow any new pressure to go to their associated brakes, the system can supply full air pressure to them, and allow the modulators to block it. The system can use the existing brake valve (under control of the BVA) to provide the supply air, and so there is no need for an ATC valve for either the driven or non-driven wheels.
While this design has met with commercial success, there is still room to improve the system.